128 
On Water and Air. 
[February 
We shall illuminate this rotating circle with a series of 
flashes, and I will show you what you would see if you were 
to observe such a rotating disc by means of a single flash of 
lightning. The effedt produced by flashes of lightning will 
be imitated here. When the disc turns round, the black 
and white will entirely disappear. [The black and white 
disc was caused to rotate, and at first illuminated by an 
uninterrupted beam from the eledtric lamp. Under these 
conditions the surface appeared of a grey tint.] You see 
that the black and white blend together in virtue of the 
persistence of the impression on the retina. I dare say 
some of the boys looking at it may turn their eyes or shake 
their heads in such a way that the disc may be resolved 
into its black and white sectors. We will now illuminate it 
with a distindt succession of flashes, and you will then see 
that it is no longer of a uniform grey. You see that every 
flash brings out the sedtors so that you can see them sepa- 
rately as if the black and white disc were standing still. 
Now if we take our liquid vein and illuminate it in that 
way, you will see it resolved into a series of beautiful liquid 
globules, and we are now going to try to illuminate a 
descending jet of water by means of a succession of flashes. 
These flashes are obtained by a disc of zinc, which is cut 
into radial slits, and is rapidly rotating before the eledtric 
lamp. You will see that as the jet of water descends a 
certain portion at the top is dark ( a n, Fig n), and the lower 
portion appears less dark. The less dark part is the un- 
steady portion. We see that at the less dark part the un- 
steadiness begins. That is the point at which the jet 
resolves itself into drops. [The jet of water was allowed 
to descend, and its shadow was thrown upon the screen.] 
Now we will try to illuminate the jet by means of a succes- 
sion of flashes. If the repetition of these flashes of light 
be at the proper rate we shall be able to resolve the jet into 
drops. [The beam from the eledtric lamp was then caused 
to flash as indicated by means of the zinc screen.] You now 
see that the jet is resolved into drops, as I predicted (Fig. 
12). I will now show you the effedt of sound upon the jet. 
I blow a certain pitch with an organ-pipe, and that enables 
the vein of water to considerably shorten its continuous 
part. You see that the top of the vein shrinks when I blow 
a note. In this way we get an apparently continuous vein 
resolved into its constituent drops. 
I have here in front of the table an arrangement which 
will enable me to show you the influence of sound upon the 
continuous portion, and also on the unsteady or discontinuous 
portion of the vein. If I turn the water on you see how 
